1. Field of the Invention
The present invention relates to an insert core to be employed in a method for manufacturing a cylinder for an internal combustion engine such as a small air-cooled two-stroke gasoline engine which is suited for use, for example, in a portable power working machine, and to a method of manufacturing the cylinder by making use of the insert core. In particular, the present invention relates to a method of manufacturing the cylinder by making use of an insert core which enables an undercut portion of the cylinder such as a scavenging port to be rationally formed on the occasion of manufacturing the cylinder by a die casting method such as a high-pressure die casting method.
2. Description of the Related Art
The cylinder of a small air-cooled two-stroke gasoline engine to be used in a portable power working machine is, as seen for instance from JP Laid-open Patent Publication (Kokai) No. 58-155114 (1983), generally formed of an aluminum alloy and constituted by an integral body including a main body having a cylinder bore formed therein for allowing a piston to be fitted therein, and a head portion having a squish dome-shaped combustion chamber formed therein, and by a large number of cooling fins projecting from all over the outer wall of the integral body.
The cylinder bore is provided with a suction port and also with an exhaust port, both of which are designed to be closed or opened by the movement of the piston. These suction port and exhaust port are disposed to face each other in an off-set manner so that they disagree in level from each other. A plurality of hollow scavenging passageways (scavenging duct), each being displaced away from these suction port and exhaust port by an angle of 90 degrees and having an inner wall of predetermined thickness, are formed along with the cylinder bore. The downstream end portion (upper end portion) of each hollow scavenging passageway (scavenging duct) is constituted by a scavenging port, thereby providing a pair of scavenging ports which are disposed opposite to each other and designed to be opened and closed by the piston. These scavenging ports are inclined somewhat upward in the direction opposite to the exhaust port of the cylinder bore.
The cylinder disclosed in the aforementioned JP Laid-open Patent Publication (Kokai) is a so-called binary fluid scavenging type cylinder where a pair of scavenging ports are symmetrically formed with respect to the longitudinal section taken along the middle of the exhaust port. Additionally, a so-called quaternary fluid scavenging type cylinder where a pair of scavenging ports are additionally provided therewith (two pairs of scavenging ports in total) is also known in the art.
As for the type of the scavenging passageway, also known in the art are a hollow scavenging passageway (scavenging duct) provided with an inner wall as shown in the aforementioned JP Laid-open Patent Publication (Kokai), a scavenging passageway having no inner wall (the side facing the cylinder bore is opened), and a scavenging passageway (scavenging duct) provided with a half-wall having a prescribed thickness as disclosed in JP Laid-open Patent Publication (Kokai) No. 2000-34924 which belongs to the same assignee as that of the present application. The last mentioned scavenging duct is featured in that it is provided at a lower portion thereof with an opening extending in the longitudinal direction of the scavenging duct while leaving a half-wall having a predetermined thickness at an upper portion thereof so as to allow an air-fuel mixture introduced into the scavenging port from the crank chamber via the scavenging duct to be contacted with a skirt portion of the piston.
When a cylinder provided with a scavenging duct in particular among the aforementioned cylinders for a two-stroke internal combustion engine is to be manufactured by a die casting method such as a high-pressure die casting method which enables cast moldings of high dimensional accuracy to be produced at low cost, the scavenging port portion of the scavenging duct which constitutes an undercut portion has been generally formed as follows. Namely, since a collapsible core cannot be employed under a high pressure, a raw cylinder body is cast-molded at first in such a manner that the scavenging port portion (constituting an undercut portion) thereof is left closed, and thereafter, this closed scavenging port portion is cut out by mechanical means (see JP Laid-open Patent Publication (Kokai) No. 58-155114 (1983)).
There is a problem however in the aforementioned method to cut out a scavenging port by mechanical means after the casting of raw cylinder body. Namely, since the space for allowing a cutting tool to be inserted into a working portion is very narrow, it is very difficult to perform the mechanical working and to enhance the working accuracy of the scavenging port. Since the performance of a two-stroke internal combustion engine is greatly influenced by the size and configuration of the scavenging port as well as by the working accuracy thereof, the aforementioned problem accompanied with the aforementioned mechanical working is very important.
It may be conceivable to manufacture a cylinder provided with a scavenging duct by a die casting method employing an insert core to be inserted into the scavenging port portion. In this case however, since part of the insert core is left to remain in the cast product, the heat conductivity thereof is deteriorated and at the same time, various problems such as the deformation or peeling of the part of the insert core may occur.
With a view to overcome the aforementioned problems, the present inventors have previously proposed a method for manufacturing a cylinder for an internal combustion engine, where an insert core is employed as described below (see JP Laid-open Patent Publication (Kokai) No. 2000-145536). Namely, according to this method, first of all, an insert core comprising a cylinder body and a scavenging port-forming portion projecting radially outward from the cylindrical body is prepared. The cylinder body has an outer diameter which is substantially the same as the diameter of bore of the cylinder to be obtained, and the scavenging port-forming portion has substantially the same cross-sectional configuration as that of the scavenging port. Then, the cast-molding of the cylinder is performed by setting the insert core in such a manner that the insert core is externally inserted over a bore-core die to obtain a raw cylinder body with the insert core remaining therein. Thereafter, the cylinder bore of the resultant raw cylinder body is subjected to boring to cut and remove the cylindrical portion of the insert core, and then, the scavenging port-forming portion of the insert core which remains in the raw cylinder body is removed by making use of a press, etc.
According to the aforementioned manufacturing method, since the aforementioned insert core is employed, it is possible to utilize a high-pressure die casting method which enables to obtain a cast article of high dimensional accuracy. Moreover, since the cylindrical portion of the insert core can be removed by way of a rough boring of the cylinder bore after the die casting, and since the scavenging port-forming portion of the insert core that could not have been removed by the rough boring can be removed by making use of a press after the die casting, it is possible to make the resultant cast article (cylinder) completely free from any residuals of the insert core. As a result, a cylinder can be manufactured in higher accuracy and at lower cost as compared with the conventional method of cutting out the scavenging port portion by mechanical means after die casting or with the conventional manufacturing method by die casting where an insert core to be inserted into the scavenging port portion is employed. At the same time, the aforementioned problems of the deterioration of heat conductivity, and troubles such as the deformation or peeling of the insert portions due to the remnant of the insert core in the cast article (cylinder) can be prevented to occur.
However, even in the aforementioned manufacturing method, the following problems are raised. Namely, according to the aforementioned manufacturing method, it is required, for the purpose of forming an suction port and an exhaust port, to prepare a core for suction port as well as a core for exhaust port in separate from the aforementioned insert core, and the die casting of the cylinder by way of a high-pressure die casting method is performed with the core for suction port being set in the bore-core die (a lower bore-forming portion thereof) and with the core for exhaust port being set in the aforementioned insert core. In this case however, since the scavenging port-forming portion, the suction port-forming portion and the exhaust port-forming portion are separately prepared, it is possible that the scavenging port, the suction port and the exhaust port are erroneously dislocated from each other due to assembling error on the occasion of setting these cores as well as due to mismatching of mold on the occasion of cast molding.
In this case, since the scavenging port, the suction port and the exhaust port are designed to be opened and closed by the piston, if these ports are relatively dislocated each other from predetermined positions thereof, in particular, if there is an error in distance in elevational direction (the elevational direction of the cylinder) among them, the opening and closing timing of these ports by the piston may become inappropriate, thus making it impossible to obtain desired performance of the engine.